Unlike the English sentence (which indicates a bit of a surprise), the Vietnamese sentence sounds natural. Additionally, depending on the context, your sentence "hả? Họ cần bao nhiêu áo khoác?" is acceptable, giving that one is surprised and trying to re-affirm the amount of coats. But, "Họ cần bao nhiêu áo khoác, à?" is not correct. "à" is equivalent to "right?" at the end of the sentence. Similarly, saying "you need how many coats, right?" does not sound right.